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Gasterophilus is a genus of parasitic flies in the family Oestridae which includes the horse bot flies. They lay eggs in the fall on the face and around the lips and noses of horses, caribou, donkeys, and similar animals, usually equines. When the eggs hatch, the larvae typically proceed to either the nasal passages or the stomach of the host animal. Those larvae which grow in the stomach are passed in the feces of the host, and they burrow into the soil to pupate. Those that grow in the nasal passages are sneezed out to pupate.
